Electric discharge apparatus



Nov. 30, 1937.

J. W. DAWSON ELECTRIC DISCHARGE APPARATUS Filed March 12, 1936 Confro/ Pofenf/a/ INVENTOR dbl??? W..Daw5077.

ATTORNEY Patented Nov. 30, 1937 UNITED STATES ELECTRIC DI John W. lDawson, East 2,100,735 SGHARGE APPARATUS McKeesport, Pa., assignor to Westinghouse Electric & Manufacturing Company, East Pittsburgh, Pa., a corporation of Pennsylvania Application March 12, 1936, Serial No. 68,475

1 Claim.

My invention relates to electric discharge apparatus and has particular relation to discharge apparatus in which a contrivance is provided for protecting the cathode from deterioration.

Hot cathode gas-filled discharge devices are now often used in rectifying and converting systems in which the current required is of considerable magnitude. In discharge devices of this type the cathode comprises an emissive surface, usually composed of the oxides of the alkaline-earth metals coated on a suitable base.

When the coating is heated, electrons are emitted therefrom and flow to one of the other electrodes,

ordinarily the anode, of the discharge device under the action of a suitable potential. It happens, however, that if the potential applied between the other electrode and the cathode is greater than a predetermined value and is applied before the cathode is heated to a sufficient- 2()' 1y high temperature, the cathode is deteriorated by the discharge which passes between the oathode and the other electrode,

In accordance with the teachings of the prior art, numerous contrivances have been provided for protecting the cathode. The practice has been to provide a time delay relay which prevents the application of the potential between the other electrode and the cathode before the cathode is heated to the proper temperature.

This solution of the problem is in many cases objectionable because of the complications in structure which it involves. Another difficulty arises from the fact that the most common time delay relays pass through their entire time cycle when they are deenergized. Accordingly, if the power in the system should fail momentarily, the discharge device will fail to supplycurrent during the entire time period of the time delay element.

In accordance with the prior art, thermal relays have also been utilized. The thermal relays are connected in series or in parallel with a filament to be protected. However, thermal elements in general require considerable energy continuously and their time of cooling is longer than that requiredfor'the cooling of the oathode. A power failure, therefore, results in a situation in which the cathode is deprived of protection unless sufficient time elapses after the power failure for the thermal relay to cool. Moreover, if the line switch after a power failure is closed before the thermal element has had time to cool, the discharge device is prematurely connected in the circuit,

It is accordingly an object of my invention to provide a simple and inexpensive system for protecting the cathode of a discharge device of the hot cathode type from deterioration by reason of premature application of potential.

Another object of my invention is to provide a system' for protecting the cathode of a hot cathode gas-filled discharge device from the deterioration by reason of premature application of potential in which the operation shall not depend on time delay relays.

An, incidental object of my invention is to provide a system for protecting the cathode of a hot cathode gas-filled discharge device, in which a control electrode is not incorporated, from deterioration by' reason of premature application of anode-cathode potential.

In the practice of my invention advantage is taken of the fact that the breakdown potential between anode and cathode of a discharge device is high when the cathode is cold but is comparativelylow when the cathode is heated to a sufliciently high temperature to safely emit current. Thus, to produce a discharge between a comparatively cold cathode and the anode of a commercial discharge device, a potential of the order of 100 volts is required. On the other hand, if the cathode is heated to the proper emissive temperature from 10 to volts is sufiicient. If the discharge device involved here is of the type having a control electrode, it is to .be remembered that the above statements are made with the. assumption in mind that the control electrode is maintained at a sufiiciently high positive potential to permit free passage of current between the anodeand the cathode.

According to my invention the discharge current at a low anode-cathode potential is utilized to operate a circuit-closing mechanism to close the main potential circuit. Since the operation of the mechanism depends on discharge current suppliedat a low potential, the main circuit is not closed until the cathode has reached the proper temperature for operation, for it is only at the proper temperature that the cathode supplies suificient current to operate the mechanism utilized.

The novel features that I consider characteristic of my invention areset forth with particularity in the appended claim. The invention itself, however, bothas to its organization and its method of operation, together with additional objects and advantages thereof, will bestbe understood from the following description of specific embodiments when read in connection with the accompanying drawing, in which Figure 1 is a diagrammatic view illustrating an embodiment of my invention, and

Fig. 2 is a diagrammatic view illustrating a modification of my invention.

The apparatus shown in Fig. 1 comprises a discharge device I having an anode 3, a cathode 5 and a control electrode 1. While my invention is applicable to discharge devices of all types, including high vacuum discharge devices, it has particular applicability to gaseous or vapor discharge devices in which the emissive portion of the cathode 5 consists of thoriated tungsten or of some base metal such as nickel, or a suitable alloy coated with a mixture of alkaline earth oxides. It is immaterial whether the cathode is heated directly or indirectly. The gas present in the discharge device is ionized when the discharge device is energized and if the cathode is not heated to a sufficiently high temperature for the potential difference impressed between the anode 3 and the cathode 5, the ions bombard the cathode and destroy the coating. My invention is chiefly concerned with the solution of the problem which arises in this connection.

The discharge device I is supplied with power through a transformer 9. When the main switch of the system is closed, heating current is supplied to the cathode 5 through a secondary section l3, and at the same time the anode 3 is connected to an intermediate point l5 of another secondary section II through. a closed movable contactor l9 and the exciting coil 2 l'of a relay 23. The secondary section I3, whereby the cathode 5 is supplied is connected to the lower terminal of the secondary section H. The difference of potential between the intermediate point l5 and the terminal point 25 and accordingly the difference of potential which is at this time impressed between the anode 3 and the cathode 5, is smaller than the difference of potential between the anode and the cathode at which the cathode would be deteriorated.

The control electrode 1 is connected to the cathode 5 through the usual system 21 that is utilized for supplying control potential to the discharge device. However, the latter system 21 is initially ineffective to function since initially the cathode 5 is also connected to the anode 3 through another movable contactor 29 of the relay 23 and a resistor 3| of suitablemagnitude to The control elecprotect the control electrode. trode 1 is thus initially in such condition that current may freely pass between the anode'3 and the cathode 5.

When the main switch H is first closed-the cathode 5 is cold and initially substantially no current passes between the cathode 5 and the anode 3. However, as the temperature of the cathode 5 rises, current begins to flow and increase in magnitude. When the cathode 5 becomes heated to the temperature at which it does not deteriorate even if the normal operating potential is impressed between the cathode 5 and the anode 3, the current flowing between the cathode and anode under the smaller potential impressed between points l5 and I1 is such that the relay 23 is energized and draws the armature 33 in an upward direction. g

An elongated rod 35 passes through the armature 33 and is secured thereto. therod 35 carries a spring 31 which is in turn secured to a third movable contactor 39 of the relay 23. The latter contactor is normally disengaged from corresponding fixed contacts 4| At its lower end.

but when the relay 23 is energized and the rod 35 is pulled upward, the movable contactor 39 engages the corresponding fixed contacts 4| and closes a holding circuit for the relay coil 2|. The armature 33 of the relay, however, does not stop moving upward when the lower movable contact 39 engages the corresponding fixed contacts 4| but continues itsupward motion albeit at the same time the spring 31 is tensioned.

The rod 35 carries lugs 43 and 45 which are disposed respectively below the movable contactors l9 and 29, but at different distances from these elements. As the armature 33 moves upward, the lower lug 43 engages its corresponding movable contactor l9 and raises it, thus opening the preliminary anode circuit through the low potential terminals l5 and 25. After this occurs, the upper lug 45 engages its corresponding movable contactor 29 and opens the connection between the anode 3 and the cont-r01 electrode 1, thus rendering the system 21, provided for normally controlling the discharge device I, effective.

The armature 33 continues its upward motion until a fourth movable contactor 41, mounted on the rod 35, engages corresponding fixed contacts 49 and connects the anode 3 to the upper terminal 5| of the secondary section |1 through the load 53. The anode 3 is now connected through the load 53 to a point 5| on the secondary section H such that the normal anode-cathode potential is supplied to the discharge device between the points 5| and 25. The discharge device is accordingly in normal operation, the control potential being supplied to it in the usual manner and the current being supplied to the load 53 through it in the customary manner.

If. a power failure should occur, the relay 23 would immediately become deenergized. First, the contactor 41 connecting the anode 3 to the secondary section l1 would then be disconnected from its corresponding fixed contacts 49. l'his would be followed by the connection of the anode to the control electrode 1 and by the Subsequent connection of the anode to the relay coil 23. Finally the lower movable contactor 39 of the relay 23 would be disengaged from its corresponding fixed contacts 4| and the holding circuit of the relay would be opened. If the power failure is only momentary the cathode would remain at the proper emissive temperature and the relay 23 would immediately pick up again. On the other hand, if the power failure is of a permanent character, the relay would remain in deenergized condition. The feature of the apparatus to be noted here is that under no circumstances can the anode 3 be connected to a dangerous point on the secondary section l1 after a power failure unless the cathode 5 is at the proper temperature for emission without deterioration.

In the apparatus shown in Fig. 2, the discharge device 55 is a full-wave rectifier comprising anodes 51 and 59 and a cathode 6| disposed in a single container. The cathode 6| is permanently connected to an intermediate tap 63 of the secondary section I1 of the supply transformer 9 through the exciting coil 64 of relay 65. Initially each of the anodes 51 and 59 are respectively connected to taps 61 and 69 of the secondary section l1 through closed contactors 1| and 13, respectively, of relay 65. The taps 61 and 69 are so located on both sides of the tap 63 that the difference of potential between the anodes 51 and 59 and the cathode BI is of a magnitude which does not produce deterioration ofthe cathode if applied before the cathode is heated to the proper emissive temperature. When the main switch H is closed the cathode is supplied with heating energy. The cathode 6| rises in temperature until it reaches a temperature such that it will not be deteriorated by the application of the normal operating anode-cathode potential. At this,time the relay B5 is energized and the anodes 51 and 59 are disconnected from the intermediate taps 61 and 69 of the secondary section H by the opening of the movable contactors H and I3 and are connected to the terminal taps l5 and 1! of the secondary section by the engagement of other movable contactors l9 and 8| of the relay with corresponding fixed cont-acts 83 and 85. At the same time the load 53 is connected between the intermediate tap 63 of the Secondary section I! and the cathode. The relay 65 being connected in parallel with the load is maintained energized by a portion of the current which is supplied through the rectifier 55. The apparatus is now in its normal energized condition.

It is to be noted that without departing from the essence of my invention the apparatus herein described may be varied considerably. In particular attention is called to the fact that the system shown in Fig. 1 may be applied to a situation in which more than one discharge device I feeds the load 53. In such a case the single relay 23 sufficies for all the discharge devices since its contactor 49 opens and closes the load circuit at a point which may be common to all the discharge devices. The relay 23 may, in the modified system, be operated from only one of the dicharge devices.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claim.

I claim as my invention:

In combination, a discharge device having a cathode, a second electrode to cooperate with said cathode and a third electrode to cooperate with said cathode, said second and third electrodes being so arranged with regard to said cathode that the discharge device operates as a full-Wave rectifier and said cathode being of the type which is heated to emit electrons and which becomes deteriorated if a potential of magnitude greater than a predetermined value is applied between it and said other electrodes when its temperature has a magnitude below a predetermined value, a source of energy for heating said cathode, two first current paths each including a source of potential of magnitude smaller than said predetermined value, two second current paths each including a source of potential of magnitude greater than said predetermined value, said two first current paths and said two second current paths being symmetrically arranged about an electrical center, means for connecting said source of energy to supply heating energy to said cathode, means for connecting said two first current paths to said cathode and said other electrodes, and means responsive to the current in said two first current paths for connecting said two second current paths to said cathode and said other electrodes after the temperature of said cathode rises above said predetermined value.

JOHN W. DAWSON. 

